The present invention relates essentially to a simplified single mooring device for mooring and loading-unloading tanker vessels from a submarine conduit for feeding or discharging a fluid, as well as a method of installing said submarine conduit and said simplified mooring device.
There are presently known various off-shore mooring and loading-unloading systems (see Journal "OFFSHORE", September 1977, pages 88-98).
For example, there is known a multiple-buoy mooring device, whereby the vessel is kept in almost stationary position by six anchors. One of its medial manifolds is connected to the submarine feed or discharge conduit through a flexible pipe resting on the sea bottom when not in use and picked from on-board the vessel at the beginning of each loading or unloading operation.
This device, however, suffers from the major drawback of being applicable only at sites where the vessel, once moored, lies permanently in the prevalent direction of the wind, of the current and of the heaviest wave. In case of variation of these elements, it may be subjected to transverse forces exceeding the capacity of its anchoring means.
There is also known a single-buoy multianchorage mooring device. In this case, the vessel rotates spontaneously around the buoy to place itself in the direction of the elements and thus reduce their action to a minimum.
At the same time it swivels a rotary joint located at the top of the buoy and which is connected to at least one floating flexible conduit ending at one of the manifolds.
Furthermore, the buoy is connected to the end of the submarine conduit by one or several hoses also lying underwater, as well as a conduit end module which is generally provided with remote-controlled valves operated from the surface.
Permanent mooring of the buoy is by means of from four to eight anchored chains, each of which must be capable of withstanding full pull from the tanker.
The device includes mechanical elements which are relatively complex and subject to wear and fatigue, such as the mono- or multi-passage rotary joint and the vessel mooring rotating head, which is a major drawback to such a device.
Also, in addition to the surface floating conduits, there are provided submarine hoses suspended from the buoy, which are strained by the movements of the latter due to swell and which subject the conduit end module to tractive forces which sometimes are considerable and of the same order of magnitude as those exerted by the vessel mooring hawser attached to the buoy.
There is also known a single-buoy single-anchored mooring device which comprises a single vertical tensioned chain which ensures multidirectional anchoring of the buoy to a gravity base or to a pile anchored base.
In this case, the rotary joint connected to the vessel by a flexible conduit partially floating at the surface is located not on the buoy but on the base. It is therefore a submarine rotary joint, and this is a major drawback to this device.
Indeed, such rotary joints with bearings, which are not actually designed and constructed for underwater operation, have a limited life, and their breakage, the time of occurrence of which cannot be forseen, will result in a pollution of the approaches, to say nothing of a shutdown of the terminal possibly for a long time for it would be necessary to await calm weather conditions before undertaking the repairs.
Another major drawback to this known buoy device lies in the fact that a collision of the vessel with the buoy, due either to a false manoeuvre or to a change of tide, may not only damage the body of the buoy but also lead to breakage of the anchoring chains, of the submarine hoses, and even cause a displacement of the Conduit End Module if the latter is not firmly anchored.
Lastly, in still another known device, the latter comprises a fixed mooring tower which, however, suffers from the drawback that it is relatively expensive and that the greater the depth of the sea bottom the higher its cost, and besides, such towers must also be provided with appropriate fenders for protection against drifting ships.